This invention relates to stabilized antenna mountings, generally used when an antenna must be supported upon a mounting which is subject to pitch and roll motions, such as a ship at sea, an offshore drilling platform, a tethered balloon, a ground vehicle, airplane, etc. While the discussion hereinafter will be with reference to a "ship", it will be understood by persons skilled in the art after having the benefit of this disclosure that some of the principles and features of the invention may be equally applicable to other mountings subject to pitch and roll motions, or any periodic vibrations or movements.
There are many applications where an antenna must be supported upon a ship at sea, or some other structure which is subject to pitch and roll motions. In the case of parabolic "dish" antennas, and other high gain antennas, pointed at satellites, it is desirable to maintain the pointing of the antenna generally in a fixed direction. Except in the rare instance of dead calm seas, an antenna mounted directly to the deck of a ship would have unacceptable pointing errors and probable loss of acquisition of the satellite under typical circumstances. In many high performance, narrow beam, military systems, a pointing error of one degree may be unacceptable. It is therefore desirable to support the antenna upon a stabilized platform.
In the past, two axes and three axes tracking antenna mounts have not been entirely satisfactory. The two axes pedestal is inherently limited to less than full hemispherical coverage by the "key-hole" effect when the target is near a line extension of the primary axis where accelerations required for corrective motions become intolerable. A three axes pedestal antenna mounting may provide full hemispherical coverage, but at a cost and complexity which is unacceptable for most commercial applications. For example, highly sophisticated control systems having closed loop servo control for each axis are typically used in such systems, along with associated rate-gyros, accelerometers, and other equipment, even at times including digital computers to perform the complex coordinate conversions. Such complex and expensive systems are not suitable for a large number of applications.
Complex four axes servo systems exist, but in order to make such a servo system sufficiently reliable, it must be expensive. The present invention achieves reliable stabilization at much less cost without servo control.
Further, the mean time between failures is generally inversely related to the complexity of a system. An acceptable mean time between failures is extremely important with antenna system usage. For example, in maritime use, a failure at sea can be costly, and at a minimum, extremely inconvenient.
In many shipboard applications, the antenna is typically mounted upon a mast or tower relatively high above the deck of the ship. This is usually desirable so that the antenna need not "look" through any portion of the ship structure regardless of the orientation of the ship. Antennas are oftentimes mounted fore or aft upon a ship so that the antenna is mounted a considerable distance from the center of the ship. As a result, the antenna will be subjected to linear acceleration forces as the ship pitches and rolls about a point which is usually located near the center of the ship. Such linear acceleration forces tend to cause a platform to tilt, and generally have a destabilizing effect upon the antenna platform.
Many proposed stabilized platforms have failed to compensate for linear acceleration forces. Many prior art patents fail to even recognize the problem of linear acceleration. This is especially true where the application disclosed in the prior art patent does not involve a ship mounted satellite antenna stabilization system. The environment of a shipboard satellite antenna stabilization system is significantly different from those disclosed in typical prior art patents. On a ship, the antenna is typically mounted far from the center of motion, usually high on a mast. The environment is characterized by significant linear acceleration forces. On a few ships, linear acceleration forces can be so great that they can cause a gyro stabilized platform that is not constructed in accordance with the present invention to destabilize and remain in a destabilized condition for a relatively long period of time.
There is a need for reliable stabilized antenna systems which have system costs that are acceptable for commercial applications. There is a significant need developing for relatively low cost, but reliable, antenna systems, particularly with the newer "L" band frequencies allocated for maritime satellite communications.
It is apparent from the above discussion that prior art antenna systems have not been entirely satisfactory. The present invention overcomes some, if not all, of the shortcomings enumerated above.
The present invention includes the feature of an acceleration displaceable mass which tends to compensate for, and offset, forces due to linear acceleration. This invention includes the feature of a stabilized platform which has an azimuth drive independent of the antenna. The azimuth drive of the antenna may be compass slaved so that the stabilized platform remains in a generally fixed orientation as the ship turns underneath, and as the antenna is turned rapidly for purposes such as cable unwraps.
The above features may be included in combination with a gimbal antenna mounting structure on a generally vertically oriented azimuth axis. The present invention preferably has a center of gravity which is located slightly below the gimbal mounting structure. The center of gravity should not be located a substantial distance below the gimbal mounting structure because to do so would provide a substantial gravity couple and make the antenna pedestal susceptible to the destabilizing effects of horizontal accelerations. The present invention features a four axes design, where two axes may be provided with a control interface while the other two axes are passively stabilized, providing a required complexity of control and reliability which is far better than with most conventional two, three and four axes systems.
The invention may include the feature of a pendular acceleration displaceable mass. A preferred embodiment should include the feature of an overall above gimbal system with a "compound pendulum" resonant frequency 10 or more times lower than the resonant frequency of the pendular acceleration displaceable mass. The addition of gyroscopes to the above gimbal system lowers the system resonant frequency greatly without the use of costly low friction, heavy load bearings, and reduces the difficulty of balancing the above gimbal system.
Specific embodiments representing what are presently regarded as the best mode of carrying out the invention are illustrated in the accompanying drawings.